What technologies could be available for an airliner entering service in 2045 that would not be ready in time for aircraft designed to be fielded in 2030? That is the question NASA asked when it awarded Boeing a year-long extension to its concept studies for “N+3”-generation airliners that could be flying in 2030-35.

In NASA's vision, N+3 is three generations on from today's 737 and 777. Boeing's “N+4” study, the final results of which were submitted at the end of February, looked another generation further into the future, targeting the 2040-50 timeframe for entry into service. What would another 15 years of technology development make possible? One answer was liquified natural gas (LNG) propulsion—in a hyper-efficient commercial aircraft already stacked with fuel-saving, emissions-minimizing advances.

The N+4 study was awarded as an extension of work completed in 2010 to identify technologies that would meet NASA's aggressive environmental targets for N+3-generation aircraft, which included a 70% reduction in fuel burn relative to the 737-800. In those Phase 1 studies—conducted by Boeing, Lockheed Martin and Northrop Grumman—several technologies were dropped from consideration because they would not be mature enough by the notional 2025 cutoff point to begin development of an N+3 aircraft.

“We saw them drop many interesting things, so in Phase 2 we gave Boeing one short task to go back and see, if they had another 15 years, what other technologies would merit some attention,” says Ruben Del Rosario, NASA Subsonic Fixed Wing project manager. The N+4 study is intended to help NASA identify which of those immature ideas the agency should start looking at now, given that it takes 20 years or more to get a new technology mature enough for the big time in the aerospace industry.

The starting point for the study was the Sugar High aircraft concept developed under Boeing's Subsonic Ultra Green Research (Sugar) N+3 project for NASA. The 154-seat Sugar High has a high aspect-ratio, low induced-drag truss-braced wing and advanced turbofan or unducted fan engines designed by study partner General Electric. In Phase 1, this configuration was found to offer a block fuel/seat saving of 39% over a 900-nm mission when powered by advanced high-bypass turbofans.

For the N+4 study, the Sugar High configuration was first updated with both airframe and engine technologies expected to be available by 2045. This resulted in an aircraft with 54% lower fuel burn than today's CFM56-powered 737-800. Although impressive, the promised saving falls short of NASA's current N+3 goal of a 60% reduction in fuel or energy consumption relative to the 737.

Boeing then added liquefied natural-gas fuel, stretching the fuselage to install cryogenic tanks forward and aft of the passenger cabin. This produced the Sugar Freeze concept, which takes the fuel-burn reduction to 57.2%, says Marty Bradley, principal investigator for Boeing Research & Technology, which led the study with the involvement of Boeing Commercial Airplanes. Adding unducted fans took the fuel saving to 62.1%, beating NASA's goal for the N+4 study.

While it might not seem an obvious choice for a future aviation fuel, LNG may offer fuel-burn and emissions reductions relative to conventional jet fuel, as well as potential cost and availability benefits, the study concludes. Despite reducing its estimate of U.S. natural gas resources last year, based on new geological data, the Energy Information Administration (EIA) in its latest annual outlook is forecasting increasing production and continued low prices until 2035.

“There is a lot of interest in LNG,” says Rich Wahls, NASA Subsonic Fixed Wing project scientist, noting that the Massachusetts Institute of Technology looked at the fuel in its own N+3 Phase 1 study (see p. 56), but decided it did not fit the timeframe. LNG offers about 15% lower carbon dioxide and 40% lower nitrogen oxides emissions than conventional jet fuel, he says.

“In the last couple of years, the identified resources have grown substantially, particularly in the U.S.,” Wahls says. Much of the natural gas is trapped in shale formations, and must be extracted by fracturing the rock. Despite recently lowering its estimate of the country's shale-gas resources, the EIA still sees the U.S. becoming an exporter of natural gas and the hydraulic fracturing, or fracking, technology needed to recover it.

“The change in natural gas reserves seemed like a correction to a number that was very uncertain in the first place. We didn't see that change in estimated reserves as impacting the future supply in a huge way,” says Bradley. “We also saw some indication the U.S. will start exporting and that the world price will come down. I also guess the U.S. will export the technology to recover more natural gas from fields around the world, increasing world supply and keeping the price relatively low in the coming decades.”

Bradley says the N+4 study suggests LNG could have a “very significant impact” on fuel burn and emissions, while offering cost and availability benefits. LNG also would be an enabler for fuel-cell hybrid electric propulsion and a step toward clean liquid-hydrogen fuel, he notes. But there are environmental concerns with methane emissions from LNG production, as well safety and infrastructure issues to be overcome. “We are recommending [that] NASA continue to investigate LNG technology,” Bradley says.

One result of N+3 Phase 1 was a strong showing by hybrid-electric propulsion technology, and for the N+4 study Boeing also looked at the potential of adding solid-oxide fuel cells (SOFC). In the turbofan configuration (see graphic above), the electricity generated powers an aft thruster that ingests the fuselage boundary layer and reenergizes the wake, reducing aircraft drag. In the unducted-fan configuration, SOFCs power an aft-mounted electric motor that helps drive the engine.

In Phase 1, GE produced a series of designs for increasingly advanced turbofans and hybrid turbine/electric powerplants under the monikers gFan and hFan, respectively. The most advanced turbofan was the gFan+, with a 77-in.-dia. fan, bypass ratio of 13:1 and overall pressure ratio of 59. The hFan, by comparison, used a 5,500-shp electric motor to provide the additional power required to drive an 89-in. fan, providing a bypass ratio of 18:1.

For the N+4 study, GE updated its engine designs with technology expected to be available by 2045, producing the gFan++ concept. This engine has a 71.4-in. composite fan with 1.46:1 pressure ratio (PR); a 1.45 PR three-stage low-pressure (LP) compressor, 28 PR nine-stage high-pressure (HP) compressor, low-NOx combustor, two-stage uncooled ceramic-matrix composite (CMC) HP turbine and seven-stage CMC/titanium-aluminum LP turbine. The engine has a slender, unitized-composite nacelle, active clearance and purge control, and integrated thrust-reverser and highly variable fan nozzle.

For the most advanced LNG-fueled, turbofan-powered Sugar Freeze, the engine has a smaller, 59.1-in. fan driven by a six-stage LP turbine and hot, high-pressure air from the HP compressor is routed via a solid-oxide fuel cell before entering the combustor. With an efficiency of around 15% and onboard reforming, the SOFC has a specific power of around 2.4 hp/lb. Electricity generated by the SOFC powers the aft BLI device, a 60.1-in.-dia., 1.45 pressure-ratio fan driven by a 3,000-hp superconducting motor with better than 99% efficiency and a specific power of around 6 hp/lb.

Boeing's N+4 study concluded that the combination of LNG fuel with hybrid fuel-cell/turbofan propulsion and the BLI thruster would reduce block fuel/seat by 60.8% over the 737-800. LNG plus the hybrid fuel-cell/unducted-fan engine would reduce fuel burn by 64.1%.

While the N+4 study is complete and the technology road maps have been delivered to NASA, Boeing is continuing work under an N+3 Phase 2 contract. This is focusing on further modeling and wind-tunnel investigation of the truss-braced wing and analysis of hybrid-electric propulsion for the Sugar Volt battery/turbine N+3 concept.

The Sugar Volt concept combines the strut-braced wing with ultra-high-bypass turbofans that can run on jet fuel and/or batteries. The removable batteries were mounted in a fairing under the fuselage, but have been moved to mid-span wing pods on the latest design. Short missions are flown mostly on batteries, and longer missions on jet fuel, giving flexibility to balance fuel versus energy use—20,900 lb. of batteries are needed for the baseline 900-nm mission, but fuel burn is reduced 63%.

Under Phase 2, the team is evaluating alternative hybrid-electric architectures. GE is updating its engine performance calculations, Georgia Tech is developing a hybrid-electric model for NASA's propulsion simulation software suite, and Boeing is updating its preferred aircraft concept. “As in Phase 1, hybrid electric continues to offer significant benefits if large battery improvements occur,” says Bradley.